Conventionally, a transmission for continuously changing a speed by using a hydraulic driving device in which a hydraulic pump and a hydraulic motor are combined is generally known. A transmission for continuously changing a speed by using a hydraulic driving device and a planetary gear speed reducer, and changing a speed stepwise by using a hydromechanical transmission and a multi-stage transmission is known (for example, see pp. 1–10 SAE TECHNICAL PAPER SERIES 941722, Hiroyuki Mitsuya et al, SAE INTERNATIONAL. 1994). In this document, when a hydraulic driving device and a planetary gear speed reducer are used, input torque is split, then one part of the torque enters hydraulic pump of the hydraulic driving device to drive a hydraulic motor, and the output force of the hydraulic motor is inputted into a sun gear. The other part is inputted into a planetary gear of the planetary gear speed reducer, and predetermined torque is outputted from a ring gear. In this situation, the ring gear is made variable in accordance with the rotational speed of the sun gear by the output force of the hydraulic motor.
In the case of using the hydromechanical transmission and the multi-stage transmission, the input torque is split, one part of the torque enters the hydraulic pump of the hydraulic driving device to drive the hydraulic motor, and the output force of the hydraulic motor is inputted into the planetary gear. The other part of the torque is inputted into the sun gear of the planetary gear speed reducer via the multi-stage transmission, and is outputted from the ring gear. In this situation, the torque is outputted as a predetermined continuous speed from the ring gear in accordance with the inputted rotational speed of the planetary gear. This transmission changes the speed from zero to a predetermined speed as a pure hydraulic driving device at the time of start of traveling, and continuously changes the speed with the hydromechanical transmission from the predetermined speed. However, this method is accompanied by shift by gears, though this method is called continuous variable transmission, and therefore there arises the problem of occurrence of gear change shock. In addition, a transmission for changing speed is required, thus causing the problem that the cost becomes high.